Steam reforming

ABSTRACT

A process for the steam reforming of hydrocarbons comprises partially oxidizing a feedgas comprising a hydrocarbon feedstock with an oxygen-containing gas in the presence of steam to form a partially oxidized hydrocarbon gas mixture at a temperature &gt;1200° C. and passing the resultant partially oxidized hydrocarbon gas mixture through a bed of steam reforming catalyst, wherein the bed comprises a first layer and a second layer, each layer comprising a catalytically active metal on an oxidic support wherein the oxidic support for the first layer is a zirconia.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/285,032, filed May 22, 2014, now U.S. Pat. No. 9,604,200, issued Mar.28, 2017, which is a Continuation of U.S. patent application Ser. No.11/915,513, filed Nov. 26, 2007, now U.S. Pat. No. 8,815,208, issuedAug. 26, 2014, which is the U.S. National Phase application of PCTInternational Patent Application No. PCT/GB2006/050097, filed May 9,2006, and claims priority of British Patent Application No. GB0510514.3, filed May 24, 2005, the contents of all of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for the catalyticsteam reforming of a hydrocarbon and in particular to the catalyticsteam reforming of a partially combusted hydrocarbon feedstock for thepreparation of synthesis gas.

BACKGROUND OF THE INVENTION

Steam reforming is widely practised and is used to produce hydrogenstreams and synthesis gas comprising hydrogen and carbon oxides for anumber of processes such as ammonia and methanol synthesis and theFischer-Tropsch process. Steam reforming may be performed over one ormore stages, for example a hydrocarbon feedstock may be reacted withsteam over a steam reforming catalyst in pre-reforming or primaryreforming steps, followed by partial oxidation of the pre-reformed orpartially reformed gas with an oxygen-containing gas, and the resultinggas stream then brought towards equilibrium over a steam reformingcatalyst. The water-gas shift reaction also occurs. The reactions may bedepicted as follows;Steam/pre-reforming C_(x)H_(y) +xH₂O⇄xCO+(y/2+x)H₂Partial Oxidation C_(x)H_(y) +x/2O₂ →xCO+y/2H₂C_(x)H_(y) +xO₂ →xCO₂ +y/2H₂Water-Gas Shift CO+H₂O⇄CO₂+H₂

In order to obtain a synthesis gas more suited to a Fischer-Tropschprocess, a primary or pre-reformed gas is typically subjected tosecondary or autothermal reforming in a reformer by partially combustingthe primary or pre-reformed gas using a suitable oxidant, e.g. air,oxygen or oxygen-enriched air in a burner apparatus mounted usually nearthe top of the reformer. The partial oxidation reactions are exothermicand the partial oxidation increases the temperature of the reformed gasto between 1200 and 1500° C. The partially combusted reformed gas isthen passed adiabatically through a bed of a steam reforming catalystdisposed below the burner apparatus, to bring the gas compositiontowards equilibrium. Heat for the endothermic steam reforming reactionis supplied by the hot, partially combusted reformed gas. As thepartially combusted reformed gas contacts the steam reforming catalyst,it is cooled by the endothermic steam reforming reaction to temperaturesin the range 900-1100° C.

Typically the steam reforming catalyst in the secondary or autothermalreformer is a nickel catalyst supported on alumina or a magnesium-, orcalcium-aluminate spinel, but precious metal catalysts can be used. Forexample EP 0206535 describes a secondary reforming process using acatalytically active metal from Group VIII of the Periodic Table,especially rhodium, wherein the catalyst support is a high purityalumina honeycomb structure.

EP-B-0625481 describes a steam reforming process in an autothermalreformer where, in order to prevent volatilised alumina refractorylining from the combustion zone in a reformer from depositing on the topsurface of the steam reforming catalyst, the steam reforming catalystcomprises an upper layer and a lower layer, said upper layer havingcatalyst particles of reduced activity for the steam reforming reaction.Because of the reduced activity of the upper layer, it will be hotterthan the lower layer, therefore preventing deposition of the volatilisedrefractory in the upper layer of the catalyst bed.

SUMMARY OF THE INVENTION

However, we have found that the volatilisation of alumina refractorydoes not occur to a significant extent unless the alumina is notproperly fired or experiences very high temperatures, for examplebecause of the burner configuration.

Furthermore, we have found that because of the high temperaturesexperienced at the surface of the reforming catalyst in a secondary orautothermal reformer, when the catalyst is supported on alumina orcalcium aluminate, significant quantities of the support may bevolatilised and deposited in cooler regions of the bed beneath thesurface. This leads to erosion of the catalyst surface, which reducesits activity, and potentially also to increased pressure drop throughthe bed. Furthermore, different flows in different parts of the bed cancause one part of the bed to vaporise more quickly. This causes thecatalyst to run hotter and the hot gases passing through the catalystcan carry a higher alumina loading as measured in mass flow terms at thesaturation vapour pressure. This then has the effect of condensingvolatilised support material out in that part of the bed just below thetop of the bed. This increases the resistance to flow in that part ofthe bed and the flow pattern at the entrance to the catalyst bed canbecome disturbed. This partial blockage can radically affect the gasflow and mixing patterns within the combustion zone, which makescombustion problems more likely.

Therefore in contrast to EP 0206535, it is desirable that the catalystat the surface of the bed is supported on a non-alumina, non-volatilesupport and in contrast to EP-B-0625481, we have found that it may alsobe desirable to provide a surface layer that has a higher activity thanthe remainder of the bed.

Accordingly the invention provides a process for the steam reforming ofhydrocarbons comprising;

-   -   i) partially oxidising a feedgas comprising a hydrocarbon        feedstock with an oxygen-containing gas in the presence of steam        to form a partially oxidised hydrocarbon gas mixture at a        temperature >1200° C. and    -   ii) passing the resultant partially oxidised hydrocarbon gas        mixture through a bed of steam reforming catalyst, wherein the        bed comprises a first layer and a second layer, each layer        comprising a catalytically active metal on an oxidic support        wherein the oxidic support for the first layer is a zirconia.

The invention further provides a bed of steam reforming catalyst,wherein the bed comprises a first layer and a second layer, each layercomprising a catalytically active metal on an oxidic support wherein theoxidic support for the first layer is a zirconia.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by reference to the examples belowand to FIGS. 1 and 2, which depict calculated vapour pressure of supportspecies or catalytically active metal species at a range of temperaturesabove 1200° C.

DETAILED DESCRIPTION OF THE INVENTION

The feedgas may comprise a desulphurized hydrocarbon feedstock such asmethane, natural gas or naphtha with a boiling point up to 200° C. whichmay be pre-heated to about 400-650° C., or may be a pre- or primaryreformed gas stream comprising unreacted hydrocarbon, steam, hydrogenand carbon oxides. The latter are preferable as it may be desirable toensure that the feed to the secondary reformer or autothermal reformercontains no hydrocarbons higher than methane and also contains asignificant amount of hydrogen, as these factors reduce the risk ofcarbon/soot formation above/on the steam reforming catalyst.

The oxygen containing gas may be substantially pure oxygen, air or anoxygen enriched air. The amount of oxygen used preferably provides an anoxygen:carbon molar ratio in the range 0.4 to 0.7:1. Steam is preferablypresent at a steam:carbon ratio between 0.5 and 2, preferably 0.5-1.5,most preferably 0.5-1. Steam may be provided by adding it directly tothe combustion zone or by mixing, either with the feedgas or theoxygen-containing gas. Alternatively, if the feedgas is a pre- orprimary-reformed gas, no additional steam may be necessary. Furthermore,if the feedgas contains hydrogen, its combustion with oxygen willgenerate steam under the reaction conditions.

The hydrocarbon in the feedgas is partially oxidised by the oxygen inthe oxygen-containing gas in the combustion zone of a suitable reformersuch as a secondary or autothermal reformer.

The partially combusted hydrocarbon/steam mixture then passes from thecombustion zone to the surface of the first layer of steam reformingcatalyst at a temperature in the range 1200-1500° C. When the partiallycombusted feedgas/steam mixture, e.g. a partially combusted pre- orprimary reformed gas, contacts the steam reforming catalyst, the steamreforming reaction, which is endothermic, cools the gas and the surfaceof the catalyst. The surface temperature of the catalyst may therefore,depending upon the conditions and activity of the catalyst, be in therange about 900-1400° C. In particular, we have found alumina or calciumaluminate volatilisation to occur when the catalyst temperature is aboveabout 1200° C. Thus, preferably the first layer reduces the catalysttemperature below about 1200° C., more preferably below about 1100° C.

In the present invention, the first layer catalyst support is azirconia. Preferred zirconia supports are stabilised zirconias, such asmagnesia-, calcia-, lanthana-, yttria- or ceria-stabilised zirconias,which are most preferably in the cubic form. Such zirconias are knownand are commercially available. Yttria-stabilised cubic zirconia is mostpreferred, e.g. a 16% wt yttria-stabilised cubic zirconia. Typicallysuch stabilised zirconias have been fired to temperatures above 1200° C.We have found zirconia-containing supports to have lower volatility thansupports comprising alumina or magnesium- or calcium-aluminate and sothe presence in the first layer of alumina or magnesium- orcalcium-aluminate is undesirable.

The catalytically active metal in the first layer of steam reformingcatalyst may be nickel or another metal suitable for catalysing thesteam reforming reaction such as cobalt, platinum, palladium, iridium,ruthenium or rhodium. Zirconia-supported nickel catalysts may containnickel in an amount between 2 and 15%, preferably 3 and 8% by weight.Thus in one embodiment, the first layer comprises nickel on zirconia,preferably 3-8% by weight nickel on a stabilised zirconia.

Preferably, the catalytically active metal in the first layer of steamreforming catalyst forms compounds having a lower vapour pressure thanthe metal in the second layer. Where the second metal is nickel,catalytically active metals that form lower vapour pressure compoundsunder comparative reforming conditions include platinum and rhodium.

To further reduce the possibility of volatilisation of the first, i.e.upper, layer of catalyst, it desirably has a higher activity than thesecond layer, i.e. the catalytic activity of the first layer is higherrelative to that of the second. By increasing the catalytic activity ofthe first layer, the endothermic steam reforming reactions take place toa greater extent and thereby act to cool the gas stream passing throughthe bed more rapidly than in the case where the layer of increasedactivity is absent. Preferably the catalytic activity of the secondlayer of steam reforming catalyst is between 1 and 95% of the catalyticactivity of that of the first layer, more preferably between 50 and 95%.

Increased catalytic activity may be achieved by providing the firstlayer of steam reforming catalyst with a catalytically active metal witha higher activity per gram for the steam reforming reaction than thecatalytically active metal of the second layer, i.e. the nickel may bereplaced by sufficient amount of a more active catalyst for the steamreforming reaction. Such more catalytically-active metals includeplatinum, palladium, iridium, ruthenium and rhodium. Rhodium isparticularly preferred as it also forms compounds with a lower vapourpressure than nickel under typical reaction conditions. Suitably activerhodium catalysts comprise 0.01-1.00% preferably 0.05 to 0.5%, morepreferably 0.1 to 0.25% Rh by weight.

A particularly preferred first layer catalyst therefore consists of arhodium impregnated zirconia catalyst, particularly a 0.05 to 0.5% wtrhodium-impregnated stabilised zirconia.

The second, i.e. lower, layer preferably comprises a nickel catalyst ona suitable refractory support. The refractory catalyst support for thesecond layer may also comprise zirconia, but for reasons of cost mayalternatively comprise alumina, calcium aluminate, titania or magnesiaor mixtures thereof. More preferably, the second layer catalyst consistsof nickel on alumina or calcium aluminate.

The first and/or second layer of reforming catalysts may be particulate,in the form of shaped units such as pellets, rings or extrudates, whichmay be lobed or fluted. Alternatively, the first and/or second layer maycomprise one or more monolithic supports such as a metal, ceramic foamor honeycomb supporting the catalytically-active metal. For example, thefirst and second layers may both comprise shaped units or may comprise alayer of shaped units over or under one or more monoliths. Preferablythe first layer is a particulate catalyst, more preferably 4-holecylinder, particularly one that is a lobed or fluted to provide a highergeometric surface area than a similarly sized solid cylinder withoutincreasing pressure drop through the layer.

Steam reforming catalysts are typically made using impregnation methodswell known to those skilled in the art of catalyst manufacture. Forexample nickel or rhodium may be provided in the first layer steamreforming catalyst by impregnation of the zirconia support with asolution of a suitable nickel or rhodium compound, for example anaqueous solution of metal acetate or nitrate, followed by heating in airto convert the compound to nickel or rhodium oxide. The nickel orrhodium oxide may then be reduced to elemental form by treatment with areducing gas such as hydrogen at elevated temperature, although it isgenerally more convenient to install the catalyst in the un-reducedoxidic form and perform the reduction in-situ by reaction with areducing gas (hydrogen and/or carbon monoxide). For example, a rhodiumcatalyst may be prepared by impregnating a stabilised cubic zirconiawith an aqueous solution of rhodium nitrate, if necessary separating theimpregnated material from the solution, drying and calcining to 400-500°C. The rhodium oxide, Rh₂O₃, is subsequently reduced in-situ. In apreferred embodiment, the rhodium is provided on the support as aso-called “eggshell” catalyst in which the rhodium is concentrated inthe surface layers of the catalyst support rather than being distributedevenly throughout the support. This provides a more efficient use of therhodium, which is expensive, compared to e.g. nickel.

If desired a layer of zirconia balls, pellets or tiles may be placed ontop of the first layer of reforming catalyst to protect the surface ofthe steam reforming catalyst from irregularities in the combusting gasflow. A benefit of providing this layer is to prevent disturbance of thesurface of the bed.

The catalytic activity of the first layer of catalyst may be furtherenhanced by providing the catalyst in a form having a higher geometricsurface area (GSA) than the second layer. The geometric surface area ofa catalyst may be calculated from its support dimensions. Increasing theGSA has the effect of increasing the surface area of catalyticallyactive metal and thereby increasing the potential for the steamreforming reaction within the first layer. Where the steam reformingcatalyst layers are particulate, this may be achieved by providing thefirst layer with particles of a smaller cross-sectional area than thoseof the second layer, or preferably by providing the first layer withparticles of the same cross-sectional area, but having one or moreholes, flutes or lobes therein. This latter method has the advantagethat the pressure drop through the bed of steam reforming catalyst maybe maintained at an acceptable value while increasing the activity ofthe first layer. Where a higher GSA material is provided as the firstcatalyst layer, preferably an inert layer of zirconia balls, pellets or,tiles is provided on top to protect the surface of the steam reformingcatalyst from the combusting gas flow.

Thus in one embodiment the first layer consists of a particulatecatalyst, preferably 0.05 to 0.5% wt Rh on a stabilised zirconia, in theform of a 4-holed pellet with a GSA of about 420 m² per cubic meter,over a particulate 5-20% wt Ni on alumina catalyst in the form of a4-holed pellet of the same GSA.

In a further embodiment, the first layer comprises a particulatecatalyst, preferably 0.05 to 0.5% wt Rh on a stabilised zirconia in theform of a 4-holed pellet with a GSA of about 420 m² per cubic meter overa monolithic alumina honeycomb supporting 5-20% wt Ni having a lowerGSA.

The thickness of the bed of the steam reforming catalyst will dependupon the activity of the catalytically active metals, the conditionsunder which it is operated and whether the feedgas is ahydrocarbon/steam mixture or a pre- or primary-reformed gas. Thethickness of the bed of steam reforming catalyst may be in the range1-10 meters, preferably 3-5 meters with the first layer preferablycomprising between 1 and 50%, preferably 3 and 25%, more preferably 5and 15% of the thickness of the bed.

It will be understood by those skilled in the art that it may be usefulto graduate the activity of the steam reforming catalyst through thebed. Therefore the second layer may comprise two or more successivelayers of a steam reforming catalyst, the third or further layers havinga lower catalytic activity than that preceding it.

EXAMPLES

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

Example 1 Calculated Example

Thermodynamic calculations on vapour pressure of different Ni and Alspecies above 1200° C. indicate that the alumina and nickel are reactingwith the reformed gas. The reaction is predominantly with the steam andthis produces hydroxides as shown below.Al₂O₃(s)+3H₂O(g)⇄2Al(OH)₃(g)Ni(s)+2H₂O(g)⇄Ni(OH)₂(g)+H₂(g)

There are other potential reactions forming other sub-oxides andhydroxides and the predicted vapour pressure of the most volatilealuminium species at 1,250° C. is 6.2×10⁻⁶ atm and for the most volatilenickel species 1.1×10⁻⁶ atm under conditions typically found in anautothermal or secondary reformer. Based on certain assumptions aboutthe partial steam pressure and temperature of the surface of the steamreforming catalyst, this could, in a reformer designed for a 4,000 mtpdmethanol plant where the autothermal reformer processes 8,000 mtpd ofreformed gas, vaporise and condense approximately 4.4 kg/day of aluminafrom the top of the catalyst bed. Over a 4-year catalyst life this couldamount to as much as 6.2 te. In the hypothetical 4,000 mtpd methanolplant, the autothermal reformer would be expected to contain of theorder of 80 te of catalyst, thus 8% of the catalyst may move in thelifetime of the catalyst charge.

Tests on alumina-supported catalysts in full-scale reformers haveconfirmed a close correlation of the plant data to the thermodynamicdata, which supports, the theoretical figures.

Furthermore, the effective vapour pressures of Pt and Rh are lower thanthe effective vapour pressure of Ni under simulated reformingconditions. Under simulated secondary reformer conditions (1 bar, 5%methane in nitrogen and steam, with a steam to carbon ratio of 3.0)nickel, platinum or rhodium catalysts on the same alumina supportmaterial were heated until the temperature reached 1200° C. When thetemperature approached 1200° C., the reforming activity, as measured bymethane slippage, was observed to decline with the nickel-containingcatalyst but was maintained for the platinum and rhodium catalysts.Analysis showed that nickel had volatilised and migrated downstream tocooler parts of the reactor.

FIG. 1 depicts the calculated vapour pressure of species formed withalumina and zirconia supports over a range of temperatures above 1200°C. at conditions typically found in an autothermal or secondaryreformer. It shows that zirconia is considerably less volatile thanalumina. FIG. 2 depicts the calculated vapour pressures of speciesformed with nickel and rhodium catalysts over a range of temperaturesabove 1200° C. It shows that rhodium is considerably less volatile thannickel.

Thus rhodium on stabilised zirconia catalysts by comparison have vapourpressures orders of magnitude lower than alumina and nickel. Based onthis, the amount of catalyst vaporisation and transfer downstream willvery small. In the case calculated above, over a 4 yr period 6,200 Kg ofNi/Al₂O₃ catalyst could be vaporised. Using Rh/ZrO₂ the amount ofmaterial vaporised would be reduced to the order of 0.06 Kg over 4years. As the active metal is also refractory, there is no loss of theactive component either, resulting in a catalyst that retains activityat the top of the bed for longer.

Furthermore, rhodium exhibits a specific activity/g between 2 and 3times that of nickel. With a 2 to 3 fold increase in specific activity,the gas temperature can be reduced from the inlet temperature of 1,250°C. to less than 1,100° C. in the layer of the rhodium/zirconia catalystbefore entering the main bed of standard nickel on alumina. A reductionin the maximum gas temperature flowing over nickel/alumina catalyst from1,250° C. to 1,100° C. will reduce the saturated vapour pressure by afactor of 4, giving a substantial decrease in the transfer of aluminadown the bed.

Calculations have been performed on three cases looking at the amount ofalumina that may be transported in an autothermal reformer. Thecalculations were based on the hypothetical 4,000 mtpd methanol plantautothermal reformer operated at the same assumed partial steam pressureand catalyst surface temperature already discussed above.

The first case, not according to the present invention, is anautothermal reformer charged with a bed of alumina lumps over a bed oflarge low activity cylinders on top of standard single hole ringcatalyst. The catalysts are all standard nickel on alumina, although thelarge cylinders have 50% less nickel making them lower activity. Thisbed would vaporise 4.4 kg/day of alumina, principally from the aluminalumps at the top of the bed and condense 4.1 kg/day of this back intothe bed, 3.1 kg/day of which would condense out in the top 0.5 m of thelarge cylindrical catalyst. This would be the most problematical part ofthe bed, slowly blocking up and increasing resistance to flow. The restof the alumina would be spread throughout the remaining 3.5 m ofcatalyst. 0.3 kg/day of alumina vapour would pass downstream to causefouling of the waste heat boiler or other heat transfer surfaces.

The second case, also not according to the present invention, is thesame autothermal reformer charged with the same bed except that thealumina lumps have been removed. This bed would vaporise 1.9 kg/day ofalumina from the first 0.3 m of cylindrical catalyst and condense 1.6kg/day of this back into the bed, 0.7 kg/day of which would condense outin a band 0.3 m deep near the top of the bed. Whilst this is a largeimprovement on the case above, it still represents a substantialtransfer of catalyst over a 4 year catalyst cycle. The rest of thealumina would be spread throughout the remaining 3.5 m of catalyst. 0.3kg/day of alumina vapour would pass downstream to cause fouling of thewaste heat boiler or other heat transfer surfaces.

In the third case, according to the present invention, a bed is chargedwith an upper layer of a rhodium-impregnated zirconia in the form of alobed 4-hole cylinder on top of standard Ni-alumina single hole ringcatalyst. In this case, enough activity is installed in the bed toreduce the gas temperature to 1,100° C. before it enters the bed ofstandard nickel on alumina ring catalyst. This bed would vaporise 0.7kg/day of alumina from the top 0.2 m of the nickel on alumina rings andcondense 0.4 kg/day of this back into the bed spread throughout theremaining perhaps 3.5 m of catalyst. This is a substantial improvementon the cases above and this bed would probably not exhibit any realpressure drop increase over a 4 yr period. 0.3 kg of alumina vapourwould pass downstream to cause fouling of the waste heat boiler or otherheat transfer surfaces.

Example 2 Comparison of the Mass Loss from Alumina and ZirconiaSupported Rh Catalysts

At an industrial scale, a primary reformed natural gas stream was fed toa secondary reformer at where it was subjected to partial oxidation in acombustion zone with an oxygen stream fed via burner apparatus disposednear the top of the reformer and passed downwards from the combustionzone to a bed of steam reforming catalyst. The oxygen:gas ratio was0.48-0.49.

The bed of steam reforming catalyst was 4 meters thick and comprised4-hole cylindrical pellets. The top 10% (0.4 m) of the bed comprisedpellets of either (a) rhodium on alumina or (b) rhodium on stabilisedzirconia. The lower 90% of the bed comprised a standard nickel onalumina catalyst. The steam reforming catalysts were provided to thereformer in oxide form and reduced in-situ. The reforming process wasoperated over an extended period for both rhodium catalysts and theweight loss measured for pellets from throughout the first top layer.

The average weight loss was then calculated.

The catalysts pellets were as follows

Hole Catalyst Diameter Length diameter GSA (as supplied to reformer)(nm) (nm) (nm) (m²m³) (a) 0.15% wt Rh₂O₃/Al₂O₃ 11 15 3 530 (b) 0.15% wtRh₂O₃/16% wt 11 12 3 560 Y₂O₃/ZrO₄

The average process conditions and weight loss/pellet in grams/day areas follows;

Primary reformer Primary Secondary Secondary Time Flow Reformer ReformerReformer Mass On- Natural Steam: Inlet Exit Loss/ line Gas carbonPressure Temp. Temp pellet Catalyst (days) Nm³hr⁻¹ ratio (atm) (° C.) (°C.) g/day Rh/Al₂O₃ 647 5407 3 39 668 990 1.4 × 10⁻³ Rh/Y₂O₃/ZrO₄ 90 62503 39 681 1014 1.6 × 10⁻⁴

The results demonstrate a lower erosion rate for the stabilisedzirconia-supported catalyst, despite the harsher temperature conditionsand higher GSA.

What is claimed:
 1. A system for steam reforming hydrocarbons, thesystem comprising a reformer vessel, the reformer vessel comprising abed of steam reforming catalyst, wherein the bed comprises a first layerand a second layer, each layer comprising a catalytically active metalon an oxidic support wherein the first layer is superposed on the secondlayer, and further wherein the catalytically active metal in the firstlayer is selected from the group consisting of platinum, palladium,iridium, ruthenium or rhodium, and the oxidic support for the firstlayer is a zirconia, the catalytically active metal in the second layeris nickel and the oxidic support for the second layer is a refractorysupport selected from the group consisting of zirconia, alumina, calciumaluminate, magnesium aluminate, titania, magnesia and a mixture thereof;wherein the reformer vessel is configured to pass a feed gas through thefirst layer and then through the second layer of the bed of steamreforming catalyst.
 2. The system of claim 1 further comprising a layerof zirconia balls, pellets or tiles placed on top of the first layer ofreforming catalyst.
 3. The system of claim 1 wherein at least one of thefirst layer and the second layer reforming catalysts are particulate, orcomprise one or more monolithic supports.
 4. The system of claim 1wherein the first layer comprises rhodium on the zirconia and the secondlayer comprises nickel on an alumina or magnesium- or calcium-aluminate.5. The system of claim 1 wherein the first layer is between 5% and 25%of the thickness of the bed.
 6. The system of claim 1 wherein the firstlayer is comprised of a rhodium impregnated zirconia catalyst.
 7. Thesystem of claim 1 wherein the first layer is comprised of a 0.05 to 0.5%wt rhodium impregnated zirconia catalyst.
 8. The system of claim 1wherein the second layer is comprised of a catalyst consisting of nickelon alumina or calcium aluminate.
 9. The system of claim 1 wherein thezirconia of the oxidic support for the first layer is a magnesia-,calcia-, lanthana-, yttria- or ceria-stabilised zirconia.
 10. The systemof claim 1 wherein the zirconia of the oxidic support for the firstlayer is a magnesia-, calcia-, lanthana-, yttria- or ceria-stabilisedcubic zirconia.
 11. The system of claim 1 wherein the zirconia of theoxidic support for the first layer is an yttria-stabilised cubiczirconia.
 12. The system of claim 1 wherein the first layer has acatalytic activity higher than that of the second layer.
 13. The systemof claim 1 wherein the second layer has a catalytic activity which isbetween 50 and 95% of the catalytic activity of the first layer.
 14. Thesystem of claim 1 wherein the first layer is on top of the second layer.15. The system of claim 1, wherein the reformer vessel further comprisesa burner apparatus, below which the bed of steam reforming catalyst isdisposed.